Formants provide honest acoustic cues to body size in American alligators

Stephan A Reber,Kent A Vliet,Kevin Torregrosa,Jim Darlington,W Tecumseh Fitch,Judith Janisch

doi:10.1038/s41598-017-01948-1

Stephan A Reber, Kent A Vliet + Show 4 more

Open Access

PDF Available

https://doi.org/10.1038/s41598-017-01948-1

Copy DOI

Export

Save

Cite

Abstract
Highlights/Summary
Full-Text PDF
Similar Papers

Abstract

Listen

In many vertebrates, acoustic cues to body size are encoded in resonance frequencies of the vocal tract (“formants”), rather than in the rate of tissue vibration in the sound source (“pitch”). Anatomical constraints on the vocal tract’s size render formants honest cues to size in many bird and mammal species, but it is not clear whether this correlation evolved convergently in these two clades, or whether it is widespread among amniotes (mammals, birds, and non-avian reptiles). We investigated the potential for honest acoustic cues in the bellows of adult American alligators and found that formant spacing provided highly reliable cues to body size, while presumed correlates of the source signal did not. These findings held true for both sexes and for all bellows whether produced in or out of water. Because birds and crocodilians are the last extant Archosaurians and share common ancestry with all extinct dinosaurs, our findings support the hypothesis that dinosaurs used formants as cues to body size. The description of formants as honest signals in a non-avian reptile combined with previous evidence from birds and mammals strongly suggests that the principle of honest signalling via vocal tract resonances may be a broadly shared trait among amniotes.

Highlights

Acoustic cues in animal vocal signals can provide receivers with a host of biologically relevant information[1]
Using Generalized Linear Mixed Models (GLMM), we investigated whether three acoustic parameters, spacing between the first two formants (F1F2)[31], dominant frequency (DF), and ƒ0 would predict the caller’s body size and controlled for the influence of caller’s sex and production location
Dorsal cranial length (DCL) was chosen as the assessment of body size for the subsequent analyses, because i) it could be measured for each recorded subject and ii) DCL and TL were found to be very strongly correlated (Spearman’s rho: n = 37, rho = 0.965, P < 0.001, see Supplementary Fig. S1)

Summary

Introduction

Acoustic cues in animal vocal signals can provide receivers with a host of biologically relevant information[1]. In birds the source signal is produced in the syrinx, an organ at the base of the trachea, and the vocal tract includes the trachea as well as supra-laryngeal cavities For both taxonomic groups, mammals and birds, adaptations to exaggerate vocal tract length have been described with some mammals lowering the larynx permanently[38] and/or during vocalisation[20, 39] and some birds developing an elongated trachea[33]. Mammals and birds, adaptations to exaggerate vocal tract length have been described with some mammals lowering the larynx permanently[38] and/or during vocalisation[20, 39] and some birds developing an elongated trachea[33] Because such anatomical constraints may remain the same for all individuals within a species, formants can remain honest cues to body size even in such cases[4, 19]. In juvenile Nile crocodiles (Crocodylus niloticus), the fundamental frequency of distress calls scales with age and adult females differentiate between the calls of larger and smaller juveniles[46]

Methods

Results

Conclusion